Hepatic transport of bile acids and inorganic electrolytes are primarily responsible for normal bile formation. Electrolyte fluxes across the hepatocytes and how they are integrated to produce bile are beginning to be sorted out. Specific ion transport mechanism and their role in bile formation will be studied in the present proposal to further increase our understanding of bile formation. The research plan will focus on the following hypotheses: 1) Active transport of C1- into the hepatocytes by a Na+ coupled C1- transport mechanism at the sinusoidal membrane followed by diffusion of C1- across the canalicular membrane may be responsible for a part of bile acid independent bile flow. The diffusion of C1- may be facilitated by calcium and/or calmodulin. 2) A C1-/HCO3- and Na+/H+ exchange at the canalicular membrane may be involved in the bicarbonate dependent bile flow and secretion of Na+ and C1-. 3) The different choleretic action of bile acids may, in part, be due to their direct effect on biliary permeability and/or electrolyte transport. The major obstacle to satisfactory study of hepatic electrolyte transport is the inability to sample bile at the site of production and to conduct electrophysiological studies. Nontheless, we can learn more of hepatocyte fluid and electrolyte transport by exploiting our growing knowledge of specific pharmacologic actions of agents which selectively inhibit one or other transport functions in other epithelia. Use of the isolated perfused liver, isolated hepatocytes and plasma membrane vesicles allows for electrolyte substitution and avoids the prohibitive systemic toxic effects of these agents in vivo and thereby allows their effects to be manifested. In this proposal we shall use 1) pharmacological agents (dibutyryl cyclic-AMP, somatostatin and furosemide) and electrolyte substitution to investigate the presence of NaC1 transport, 2) electrolyte substitution and pharmacological agent (a disulfonic acid stilbene derivative, SITS) to investigate the role of C1/HCO3- exchange in bile formation and 3) stanard measure of biliary permeability to assess the mechanism of differing choleretic properties of bile acids. While pursuing these objectives, we shall also investigate the possible role of calcium and calmodulin in bile formation and cholestasis.